Now, let's consider the case without mass. The expressions for the massless
scalar field can be obtained from the limit \(m\rightarrow 0\) . For this goal, we
use the behavior of the Bessel functions for small arguments [1]}
\(z\rightarrow 0\Rightarrow K_{\nu }\left( z\right) \sim \frac{1}{2}\Gamma \left(\nu \right) \left( \frac{z}{2}\right) ^{-\nu }.\) 
